Letter 11: Be the gatekeeper of your heart and question every thought before letting it enter.

If the possession of blessed freedom from passion is desirable to you, and if you want to receive the holy knowledge of Christ, then practice with your whole soul everything that belongs to the finding of life. At the times of prayer, ask the Lord for wisdom, which by his grace makes you free from passion and reveals to you the method by which the demons deceive. Without this wisdom, the soul directs nothing rightly, and the mind is not quick in the contemplation of the one who made it. The mind must be bold against its adversary, as blessed David shows when he takes up words from the demons' mouth and speaks against them. When the demons say, "When will he die, and when will his name perish?" he answers, "I shall not die, but live, and tell the works of the Lord." Again, when the demons say, "Flee to the mountains like a sparrow," he says, "He is my God and my Savior, my helper; I shall not be shaken further." See how these voices answer one another. Love the victory, become like David, and watch yourself. Be the gatekeeper of your heart, and do not let any thought enter without questioning it. Ask each thought: Are you one of ours, or one of our enemies? If it belongs to us, it will fill you with peace. If it belongs to the enemy, it will trouble you with anger or shake you with desire. Such are the thoughts of the demons. You must test the condition of your soul at every moment: whether your will failed to prosper when you expected something in hope; whether you were harmed in some matter, or asked and did not receive; whether you were honored or dishonored, reviled or praised. This is according to the word of the blessed apostle: through the weapons of righteousness on the right hand and the left, through dishonor and glory, through slander and praise, as deceivers, so that you may lead your vainglory astray, and yet as true, so that the knowledge of Christ may be impressed in you; as unknown, that you may possess humility, and as well known, that you may possess boldness in Christ; as poor in possessions and enriching many in the knowledge of our Lord; as having nothing and yet, through faith, possessing everything. These are signs of freedom from passion, full of every blessedness.

Greek retroversion from Syriac transmission (Frankenberg 1912, TAN/TEI CC BY 4.0): ει ποθητον σοι μακαριας απαθειας το κτημα και την αγιαν Χ. γνωσιν δεχεσθαι θελεις παντα τα εις την της ζωης σου ευρεσιν προσηκοντα επιτηδευσον (πραττε) απο πασης της ψυχης σου και προσευχης καιρων αιτει σοφιαν παρα του κυριου δια της αυτου χαριτος σε απαθη καθιστασαν τε και την της δαιμονων πλανης μεθοδον σοι καταμηνυουσαν· ανευ γαρ αυτη η ψυχη ον ουδενι ευθυνει ο δε νους ουκ οξυκινητος εν τηι του αυτον ποιησαντος οψει. δει τοινυν αυτον ειναι θαρραλεον κατα του αντιδικου αυτου ως αποδεικνυει ο μακαριος Δαυιδ τιθεις φωνας ως απο των δαιμονων του στοματος και αυτοις αντιλεγων· οταν γαρ οι δαιμονες ειπωσι ποτε αποθανειται και απολειται το ονομα αυτου λεγει και αυτος ουχ αποθανουμαι αλλα ζησομαι και εκδιηγησομαι τα εργα κυριου · και παλιν των δαιμονων ειποντων μεταναστευου επι τα ορη ως στρουθιον λεγει αυτος αυτος ο θεος μου και σωτηρ μου αντιλημπτωρ μου ου μη σαλευθω επι πλειον . ιδε δη φωνας αλληλαις αντιρρητικας και αγαπα νικην και ομοιωθητι Δαυιδ και αποβλεψον εις σαυτον· ισθι θυρωρος της καρδιας σου και μη αφες λογισμον εισιεναι ανευ ερωτησεως, ερωτησον ενα εκαστον των διαλογισμων λεγων ημων ει η των εναντιων ημων και ει οικειος εστιν ειρηνην σε εμπλησει ει δε εναντιος θυμωι σε διαταραξει η επιθυμιαι σε ανασαλευσει. τοιουτοι εισιν οι των δαιμονων λογισμοι· προσηκει τοινυν σε δοκιμασαι παντι καιρωι της ψυχης σου την καταστασιν ει τι προσδοκησαντος ως επ' ελπιδι ουχ ευωδωθη το θελημα σου και ει εν τινι εζημιωθης η αιτησας απετυχες η εδοξασθης η ητιμηθης η ωνειδισθης η ευφημηθης, κατα το ρημα του μακαριου αποστολου· δια των οπλων της δικαιοσυνης των δεξιων και αριστερων δι' ατιμιας και δοξης δια δυσφημιας και ευφημιας, ως πλανοι, ινα αποπλανηις σου την κενοδοξιαν, και ως αληθης παλιν, ινα εν σοι η Χ. γνωσις εντυπωθηι, ως αγνοουμενος κεκτησο ταπεινοτητα και ως επιγιγνωσκομενος κεκτησο παρρησιαν εν Χ, ως πτωχος κτηματων και πολλους πλουτιζων εν γνωσει κυριου ημων, ως μηδεν εχων δια δε πιστεως παντα κατεχεις. ταυτα εστι σηματα απαθειας πασης μακαριοτητος πληρους Syriac transmission available in the linked TAN/TEI source. The complete corpus is Syriac-transmitted; Greek survives only fragmentarily, so this display text is a retroversion witness.